First, a couple of additions to yesterday's "how to be a good con guest" post:
Rule #9: Don't introduce yourself or start off talking on a panel by saying that you don't know why you're on that panel.
If you don't know why you're on a panel, the time to ask is as soon as you get your schedule from the convention (usually at least a couple of weeks in advance), and the person to ask is the person who's in charge of programming. You never know, there could be a method to their madness. They may want you to give the outsider viewpoint, be the voice of dissent or provide a different perspective. Someone else on the panel may have listed you among the people they want to be on panels with, and this is the only panel where your schedules line up, or this could be the only place they could put you on a panel with someone you said you wanted to be with. If you don't want to be on a panel you've been assigned, if you're uncomfortable with the subject matter or just think you don't know enough to be a decent panelist, it's okay to turn down the panel. But do this as soon as you get your schedule so the convention has time to find someone else to take your place and maybe even find something else for you to do. It's better for everyone that you don't do a panel you have no interest in because it keeps you from looking uninformed and it means the audience will get an interesting panel made up of knowledgeable speakers. That face time will not significantly make or break you, and doing a panel you aren't really suited for may do more harm than good. Besides, if you have no interest in that subject, there's a good chance that the target audience for what you write will have little interest in that subject and won't even be at that panel. If you still decide to do the panel, then for goodness sake, don't say you don't know why you're on the panel. You can be a good listener, you can smile and nod and you can even ask the other panelists questions. The moment you say you don't know why you're on a panel, you totally destroy your credibility. Why should the audience even listen to you?
To prove that I practice what I preach, I turned down a panel at Worldcon, where panel slots can be hard to come by, because I knew I'd have nothing to contribute to that panel. When it comes to programming, quality is far more important than quantity. It's better to be on one really good panel where you have something to contribute and can sound witty and intelligent than to be on a bunch of panels where you don't really have anything to say.
Corollary to Rule #9: Don't do the "I don't know why I'm on this panel" thing as a form of false modesty or a joke -- as if you're really saying there should be no doubt why you're on the panel because obviously everyone knows you're the expert on the subject. If the audience knows who you are well enough to understand why you do, in fact, belong on the panel, the joke is unnecessary, and if they don't, then it falls flat and they wonder why you didn't just tell the programming people you didn't belong on the panel. Or you look insecure, like you're fishing for compliments (as though the rest of the panelists are supposed to hurry to tell you that of course they couldn't have this panel without you). About the only time when pretending you don't know why you're on a panel might be funny would be if the panel was specifically about your work or about something you're widely known to be involved in (like if it's a panel about a TV series you write for or were otherwise involved with).
This next is probably not so much a rule as it is a tip. Because they have not yet invented the Auto-Con 9000 robot to run conventions, they are still run by human beings who are fallible. It's a good idea to check the schedule sent to you against the schedule in the program posted at the con's web site to make sure they match, and then let the con know if something doesn't match. That will decrease your chances of entirely missing a panel you were supposed to be on that wasn't on your schedule or of feeling silly when you show up for a panel your schedule says you're on that doesn't list you in the program.
Take time to fill out the programming questionnaire thoroughly, and let the con staff know about any scheduling limitations you have as soon as you're aware of them.
If you're in a group reading, try to treat it like a panel and arrive before it starts and stay to the end rather than coming or going while someone else is reading.
Once you get to be a bigger name, be aware that the size (and maybe enthusiasm) of your fanbase means that they may be infringing on others. You aren't necessarily responsible for the behavior of your fans, but if you notice that their efforts to follow you are being detrimental to other guests, it's nice to say something about it to encourage them to behave otherwise. That includes coming and going during readings, behavior during readings when you aren't the one reading, blocking access to other authors during autograph sessions and mobbing the table after a panel discussion so the next panel can't get started on time.
Finally, be aware that if I notice bad author behavior, there's a good chance I'll have more material to post ...
And now I must get groceries before getting down to work.